1. Field of the Invention
The present invention relates to a method for producing semiconductor wafers by a repeated sequence of grinding the end face of a monocrystal using a grinding tool and cutting a semiconductor wafer from the monocrystal using a cutting tool, a grinding abrasion of a specified depth being produced during grinding, and the semiconductor wafer being cut in a cutting plane which is as parallel as possible to the ground end face.
2. The Prior Art
The grinding of the end face of the monocrystal produces a first flat side face on the semiconductor wafer. After the semiconductor wafer is separated from the monocrystal, this first flat side face is used as a reference face and the opposite side face is ground parallel to this reference face. The desired result is a semiconductor wafer having flat and parallel side faces. U.S. Pat. No. 4,967,461 describes how the grinding of the end face of the monocrystal eliminates unevenness which is attributable to deflections of the saw blade during the cutting of a semiconductor wafer and which would result in warped semiconductor wafers. Conventionally, the saw blade of an annular saw is used as the cutting tool. While the saw blade is working through the monocrystal, sawing forces occur which deflect the saw blade from the intended cutting plane situated parallel to the ground end face of the monocrystal. The actual cutting plane is therefore not completely flat, but slightly curved and this unevenness is reflected in the quality of the end face of the monocrystal. Before the next semiconductor wafer can be cut, the end face has to be ground flat again.
The repeated sequence of grinding the end face of the monocrystal and cutting a semiconductor wafer from the monocrystal applies stresses both to the grinding tool and to the cutting tool. The condition of the grinding tool can be determined with a perthometer. However, this requires a fairly long shutdown time for the grinding machine since the investigation is carried out on the grinding tool and much time has to be expended on dismantling and reassembly and also on the readjustment of the grinding tool. Wear phenomena or changes in operating parameters may result in the deflection behavior of the saw blade altering over the course of time. These changes may alter the cutting pattern when a semiconductor wafer is being cut, and lead to a different cutting pattern from the cutting pattern that results when a subsequent semiconductor wafer is later being cut. Sensors were therefore developed to enable alterations in the cutting pattern to be observed and followed even during the cutting operation. Suitable sensors are described, for example, in U.S. Pat. No. 4,991,475.
When the end face of the monocrystal is being ground, an attempt is made to keep the grinding abrasion to a minimum. The grinding abrasion is specified in a length unit. It corresponds to the distance between the ground end face and the highest material elevation on the unground end face. If the resulting grinding abrasion is too high, material is wasted and the yield of semiconductor wafers per monocrystal is reduced. If the grinding abrasion is too low, the unevenness on the end face of the monocrystal is not adequately removed and the semiconductor wafers produced are defective.
A specified grinding abrasion can be set very accurately with the aid of the feed unit which moves the grinding tool and/or moves the monocrystal for the purpose of grinding. A predetermined required grinding abrasion may, however, soon prove to be too low, for example, because the magnitude of the deflection of the saw blade has increased during the course of the wafer production. On the other hand, the saw gap in the monocrystal due to the saw blade may become thinner, for example, as a consequence of wear of the saw blade. This wear will cause the result that, if the original grinding abrasion is maintained, more material would be ground from the end face of the monocrystal than necessary.
Since the reference grinding abrasion and the necessary actual grinding abrasion may be different, there is an urgent requirement to be able to determine the grinding abrasion at least from time to time. However, this is difficult to do. One known method is to cut a semiconductor wafer from the monocrystal at regular intervals without the end face of the monocrystal being previously ground beforehand. The grinding abrasion is then measured directly from the thickness of the semiconductor wafer, provided the cutting conditions have not changed since the penultimate cutting of a semiconductor wafer. Constant cutting conditions may be assumed if sensors do not reveal any substantial alteration in the cutting pattern during the cutting of two consecutive semiconductor wafers. A disadvantage of this known method described above, in particular, is that the semiconductor wafer investigated becomes a reject and reduces the yield, because it lacks the flat reference face and cannot be used as intended.